Progesterone is the major female sex hormone, and is responsible for reproductive-related activities such as breast glandular development, the endometrial aspects of the menstrual cycle, and the establishment and maintenance of pregnancy. In addition, progesterone also directs pregnancy support physiology including changes in carbohydrate, protein and lipid metabolism, thermoregulation, sodium reabsorption in renal tubules and the reduction of alveolar and atrial carbon dioxide partial pressures (pCO2). Progesterone is secreted in large amounts by the corpus luteum and by the extracellular conversion of cholesterol, cholestryl esters, adrenal steroids, prenenolone and pregnenolone sulfate. Small quantities are also secreted directly from the adrenal glands.